(1) Field of the Invention
The present invention relates to an image capturing apparatus for such as a digital camera. More particularly, the invention is directed to adjustment of an amount of light for producing high-quality images.
(2) Related Art Statement
With respect to an image capturing apparatus using an image-forming optical system such as lenses, particularly a digital camera, the quality of images is deteriorated due to influence of diffraction at an edge of a diaphragm, when an aperture stop is reduced in photographing a subject at a high brightness. For this reason, light extinction means using an ND (neutral density) filter is often employed in a high-grade camera so that the diaphragm need not be throttled beyond a predetermined level. On the other hand, tech-savvy camera users desire cameras which can realize photographing effects with the diaphragm. That is, such cameras are desired, which can reduce the diaphragm aperture to a smaller degree so as to ensure a depth of the subject field, whereas its diaphragm aperture can be largely opened to appropriately defocus a background or a foreground. For realizing the above object, plural-stage diaphragms are often used in high-grade cameras.
In order to satisfy the above two demands, it is effective to mount a multistage diaphragm using the ND filter in the camera. FIG. 4 shows an outer appearance of a shutter unit 14 with a multistage diaphragm using an ND filter 35 in combination. In FIG. 4, the multistage diaphragm 13 is generally constituted by a plurality of diaphragm blades, which are driven by driving cams with a pulse motor, thereby realizing a desired aperture diameter. Since the pulse motor is used as the driving source for the multistage diaphragm 13, the aperture diameter is varied stepwise. The multistage diaphragm is usually designed such that the diaphragm aperture diameter varies every ½ AV or ⅓ AV per one pulse. According to the ND filter 35, a sheet-like filter is ordinarily bonded to a blade member as a support, and the amount of light is adjusted by moving the vane members onto or away from the optical axis.
The light amount may be adjusted by using the ND filter having such a size as to cope with the released state of the diaphragm. However, since the ND filter is costly, the ND filter preferably has as small a size as possible. That is, if an exposure diagram as shown in FIGS. 10 and 11 can be attained, it is preferable that the ND filter is used in the state that the aperture stop is reduced to some extent, and the smaller ND filter copes with the situation. As the ND filter has a small size, a mechanism for retaining and driving the ND filter can be made smaller. The dimensional relationship between the released diaphragm aperture diameter and the ND filter 35 is set in the example of FIG. 4 such that the ND filter 35 can be used in the state that the diaphragm 13 is throttled from the released aperture value by 1 AV or more.
As to the multistage diaphragm combined with the ND filter to be used in the case of the high-grade camera as mentioned above, the camera can be so constructed that the ND filter can be used from the released state or the near-released state of the diaphragm. Thus, in some area, there are two patterns with respect to a given light extinction amount required between the released diaphragm aperture value and the minimum aperture value: one pattern being a case in which throttling is effected with the multistage diaphragm alone and the other being a case in which the amount of the light is reduced by using the diaphragm and the ND filter in combination. FIG. 5 shows an example thereof In FIG. 5, shaded regions indicate that F-values and AV values in use of the multistage diaphragm alone overlaps with those in using the multistage diaphragm and the ND filter in combination, respectively. FIG. 6 is an exposure diagram showing the above. In FIGS. 5 and 6, the AV values with no ND filter overlaps with that with ND filter over the six stages from F5.0 to F9, respectively. The released state of the multistage diaphragm is indicated by an encircled figure “1”, whereas the minimum aperture stage is indicated by an encircled figure “12”. Encircled figures indicate specific aperture stages, respectively. In FIG. 6, lines corresponding to uses of the ND filter in multi-stage No. 9 to 12 of the diaphragm are also drawn. In the actual camera, the light amount-reduced states over these four cases are not used, because use of around the LV1 to LV8 is sufficient.
When the same light amount-reduced state is attainable with respect to a specific luminance of a specific subject by any one of the use of the multistage diaphragm alone and the use of the multistage diaphragm and the ND filter in combination, it is a problem as to which is appropriate, the use of the multistage diaphragm alone or the use of the multistage diaphragm and the ND filter in combination. FIG. 7 compares advantages and disadvantages between the multistage diaphragm based light amount reduction system and the ND filter based light amount reduction system. What is first to be considered in case of the cameras lies in how to obtain better quality images. In that case, the problem to be considered is degradation in the quality of the images at the time of small aperture as mentioned above. This is the greatest reason why the ND filter is used as the light reduction or extinction means. The ND filter is used to remove adverse effects due to the small aperture stop. FIG. 8 shows an example of the relationship between the aperture stop and the image quality (IF). As shown in FIG. 8 when the aperture stop is throttled by around one stage, the MTF is maintained to some good degree, but the MTF dearly decreases when the aperture is more throttled. It is preferable to use the light extinction system with the ND filter so as to prevent such degradation of the MTF.
On the other hand, the reason why the multistage diaphragm is preferably used is that the depth of subject field can be set according to a photographer's intention. In order to attain an appropriate focus over a wide range from a close view to a distant view, a control method (multistage diaphragm) in which the aperture is reduced at multi-stages is required.
Next, with respect to a problem such as a ghost image or reduction in the peripheral light amount, there will be a difference between the multistage diaphragm light extinction-based system and the ND filter light extinction-based system. The ND filter system is more disadvantageous for the ghost image, because a reflecting surface increases. Such a ghost image is reduced by throttling with the multistage diaphragm when the ghost is shielded by the aperture stop. With respect to the problem in which the peripheral light amount decreases, the light amount is made nearly uniform to mitigate the problem, when the aperture stop is made smaller, whereas the problem is not reduced by the ND filter (See FIG. 9).
With respect to the power consumption, there is a difference between the multistage diaphragm light extinction system and the ND filter light extinction system. The multistage diaphragm is driven by a pulse motor in many cases, and the pulse motor having two coils is disadvantageous in terms of the power consumption, that is, the power consumption is larger. On the other hand, the ND filter has only two positions: an advanced position onto the optical axis and a position retracted from the optical axis, so that a moving magnet or the like having one coil can be used. Thus, the ND filter is advantageous in terms of the power consumption, that is, the power consumption is smaller. However, in order to reduce the power consumption, it is the best way to keep the unmoved state as much as possible.
In light of the above-mentioned prior art techniques, there has been proposed an image pickup apparatus in which an optimum camera controlling method is selectively switch controlled under consideration of the advantages and the disadvantages of the multistage diaphragm light extinction system and the ND filter light extinction system for example, JP 2003-134393A). More specifically, the apparatus comprises an exposure controller which is to perform the control by using an aperture stop and a transmittance controlling means in combination, wherein the exposure controller has a first operation mode in which the aperture stop and the transmittance controlling means are controlled to have precedence on the depth of subject field. In addition to the first operation mode, this exposure controller has a second operation mode in which the aperture stop and the transmittance controller are controlled to have precedence on the resolution. Thereby, the apparatus makes the control by switching between the first operation mode and the second operation mode, depending upon the photographing condition.